1. Field of the Invention
The invention relates to a fan structure and, more particularly, to an axial-flow fan structure capable of directing an air flow sideways.
2. Description of the Related Art
A fan is used to direct air to flow and thus to dissipate heat, and better air directing efficiency results in better heat dissipating efficiency. Usually, there are two types of fans; namely, axial-flow fans and centrifugal fans. In an axial-flow fan, an impeller is rotated to direct an air flow in a direction parallel to the central axis of the impeller. Having advantages of simple structures, lower manufacturing costs, and higher air flow output, axial-flow fans are widely used in air conditioners and heat dissipating devices such as heat-dissipating fans in computer systems. However, the use of axial-flow fans is limited to systems with lower system resistance because the air flow output is restricted, and the stall problem occurs when the system resistance of an axial-flow fan gets larger.
Referring to FIG. 1, the vertical axis and the horizontal axis represent the static pressure and the air flow of an axial-flow fan, respectively. A characteristic curve A of a conventional axial-flow fan intersects a system resistance curve (SRC) C at a point OP, which represents an operation point of the fan. At the point OP, the air flow and the static pressure are Qop and Pop, respectively. On the other hand, since the boundary layer separation of air may occur on the blade surface when the rotating speed of the fan reaches a certain speed, the air flow cannot be increased even though the rotating speed of the fan is higher. At this time, the variation of the static pressure is smaller, which is known as “stall” of the fan and represented by a line DE in FIG. 1.
Axial-flow fans have advantages of simple structures, lower manufacturing costs, and higher air flow output, and thus play important roles in many engineering applications. However, to solve the stall problem of axial-flow fans, the focus has always been on the design of angles, lengths, and number of the blades; and yet no simple and effective solutions have been found for the problem.
Referring to FIG. 2, an axial-flow fan 1 includes a frame 11, a hub 12, blades 13 and a stator base (not shown). An edge portion 131 of each blade 13 is fastened to a side surface 121 of the hub 12. When the fan 1 rotates, these blades 13 guide the air above the blades 13 to flow downward in an axial direction of a rotation shaft (not shown). For this axial-flow fan 1, however, since the hub 12 has an almost equal height to that of the frame 11, the blades 13 are not able to guide all of the air above the hub 12, and thus a region above the hub 12 becomes ineffective.